I am looking for an effective way to sound proof our walls from road noise. We have an old house with a 4 inch empty wall space. We just had our walls insulated with poly master foam which has a reasonable STC value of 44. It did not do a damn thing and I feel like I just wasted quite a buck. It looks like insulation even designed for sound reduction will not do the job well with low frequency noise. I am considering double 5/8th drywall on the inside wall. They make drywall specially designed for sound reduction that has an STC rating of 55. I think two layers might do something. I also do not know how much faith to put in these STC values given my recent experience with the insulation. I know that most masonary has poor STC vaules; however, looking at the highway barriers and having a friend that lives next to one maybe these specially designed brick might be a good way to go. I certainly could use some adivse myself or I will have to move from this dream waterfront property I bought less than a year ago.